The present invention relates to solid biocide dry blend concentrates and methods of making such concentrates.
Resin compositions are protected against fungal or bacterial attack by incorporating a microbiocide therein to prevent the deterioration of articles formed from the resin composition due to microbiological attack on the susceptible portion of the components of the resin system. In order for the microbiocide to be effective in the resin composition, it is necessary that it be compatible therewith and uniformly dispersible in the resin composition to avoid forming resin composition portions free of the microbiocide which would be susceptible to attack.
In the past, microbiocide compositions have been added to resins either as a powder or as a liquid composition. To assure compatibility and adequate dispersibility of the microbiocide, it was believed necessary to add the microbiocide with a liquid carrier, such as a plasticizer for the resin. The presently employed procedures usually involve first mixing the microbiocide in a liquid carrier which solubilizes or disperses the microbiocide uniformly, followed by mixing the liquid composition with the final resin composition. The liquid solvents or dispersants employed are those such as plasticizers which do not degrade the properties of the final resin product when employed at moderate concentrations. Unfortunately, the solubility of many of the commonly used microbiocides in common liquid resin additives is quite low. Therefore, it is difficult to incorporate a sufficiently high concentration of the microbiocide with the resin while avoiding an undesirably high concentration of the liquid carrier. Also, this procedure imposes restrictions on the choice of plasticizer to be used in the final resin composition. In addition, it is desirable to avoid using plasticizers with some thermoplastic resins such as polyurethanes and polyolefins.
Alternatively, it has been proposed to add the microbiocide directly to a formable resin composition at the low effective concentration which prevents microbiological attack. However, this procedure has proven to be unsatisfactory since the needed concentration of microbiocide is quite low, generally less than about one wt. % and usually between about 200 and 1000 parts per million. If the microbiocide were to be employed in the resin at higher concentrations, the toxicity of the final product made therefrom may be dangerously increased. Therefore, if this procedure is employed, the processor must continuously carefully weigh small amounts of microbiocides to be added to the final product. Since most microbiocides available for protecting resins are powders, continuous handling of a fine-powdered solid which can easily be dispersed in air presents a major toxicological problem to the personnel working in the immediate area. To eliminate these toxicological problems, major changes would be required in presently employed commercial plastic processing techniques which would render them expensive and commercially unfeasible. For this reason, the commercial processor utilizes the microbiocide in a liquid carrier which is somewhat less innocuous than the microbiocide per se. In addition, in order to attain homogeneous dispersion of these low concentrations of microbiocide into the resin, it is necessary to extend the mixing time of the resultant composition. Furthermore, mixing of these resin compositions containing low concentrations of microbiocides results in a substantial portion of the microbiocide being coated on the surface of the mixing apparatus rather than being homogeneously dispersed throughout the resin.
One solution to many of these problems is described in U.S. Pat. No. 4,086,297. The '297 patent describes a solid microbiocide concentrate composition comprising a homogeneous mixture of a solid thermoplastic resin and from 1 to 80 wt. % of at least one microbiocide which is insoluble in water. The microbiocide is readily dispersible or soluble in the resin at temperatures sufficiently high to permit plastic manipulation of the resin and the dispersion or solution of the microbiocide is sustained indefinitely upon cooling to ambient temperature while the diffusivity of the microbiocide in the resin under such conditions becomes vanishingly small. The microbiocide retains its microbiocidal activity in the resin and does not degrade or react with the resin in which it is dispersed. The composition described therein provides a convenient non-toxic dosage form of the microbiocide which is subsequently mixed with a second thermoplastic resin at a concentration of about 0.5 to 15 wt. % to obtain a homogeneous resin composition containing an effective amount of the microbiocide.
Although the '297 patent describes compositions acceptable in most instances, certain disadvantages remain. The process used to prepare the compositions of the '297 patent includes heating the microbiocide and resin to melt and soften the resin and to form a homogeneous mixture. The homogeneous mixture is subjected to shear forces and heat in a suitable apparatus and then formed, such as by extrusion, milling or calendering. After cooling, the formed composition is broken up into small particles, e.g., pellets, thereby permitting its incorporation into other thermoplastic compositions to provide an appropriate concentration of microbiocide.
The particles resulting from the above process are not suitable for incorporation into plastisols. Nor are such particles usable when screening to filter out impurities such as paper, etc. is performed in preparing a dry blend prior to extrusion. In addition, the heating, mixing and cooling steps of the process of the '297 patent are relatively high in terms of energy costs. Finally, the heating step of the process of the '297 patent means that microbiocides which are heat sensitive cannot be used therein.
Another attempt at solving the problem of handling dusty additives which may be toxic is set forth in U.S. Pat. No. 4,025,690. The '690 patent describes a process for preparing pigment additives for plastic materials in which the dusty additives are absorbed by a polymer or copolymer of vinyl chloride in granular form. The process consists of treating the additives and polymer or copolymer in a mixer while raising the temperature to obtain absorption of the additive by the particles of the polymer or copolymer. The mass is then cooled to ambient temperature. While appropriate for some additives, powdery additives, such as microbiocides, continue to be dusty even after absorption, and much of the biocide remains on the surface of the resin where it can flake off and cause excessive dusting during handling. The biocide can be readily seen on the surface of the resin particles by microscopic examination.
Accordingly, a need exists for a microbiocide concentrate which is useful in plastisol formulations, is more simply prepared than prior art concentrates, and allows a broader range of microbiocides to be employed, while retaining the reduced toxicity and ease of handling of the compositions of the '297 patent.